The present invention relates to an electronic device using a functional device whose function is achieved at a particular temperature or whose performance is enhanced at a particular temperature and also using a thermally isolated element including an electrical-to-thermal transducer.
Functional devices, such as, a semiconductor device and an electronic device are generally used at room temperature without performing particular temperature control.
Of course, when functional devices generate large heat, cooling means, such as, a cooling fan, a water cooling pipe or a heat pipe is used to prevent overheating.
However, even in this case, the purpose of the cooling means is to be operable near room temperature. That is, the common sense in the art is that functional devices are operated at a temperature near room temperature.
To this end, a superconducting device, such as, a Josephson device, has been used for application, such as, observation of stars in which low noise performance is particularly required. However, the superconducting device is not widely used because it does not operate at room temperature.
It is well known that semiconductor devices generally exhibit their highest performance not at room temperature but at lower temperatures. More specifically, semiconductors generally have greater electron mobilities at low temperatures, and thus semiconductor devices can operate at higher speeds at low temperatures.
In optical devices, such as, a light emitting device and a semiconductor laser, the luminous efficiency can be increased to a great extent by reducing the operating temperature.
The above-described enhancement of performance at low temperatures results from a reduction in phonon scattering. In most semiconductor devices, as described above, their potential is not fully exploited.
Most of functional devices are used at a temperature near room temperature, as described above. However, the room temperature varies depending on locations and environments. Room temperature in some area is extremely low but extremely high in another area.
In extremely low-temperature areas, the outside air temperature becomes lower than -50.degree. C. On the other hand, the temperature can be higher than 80.degree. C. in a parked car or on a telephone pole in summer. As described above, room temperature, at which functional devices are operated, varies within such a wide range.
In other words, in order to fully use the potential of functional devices or to assure the operation of functional devices, it is required that functional devices be operated in a thermal environment which is controlled within a proper range.
For example, superconducting devices essentially need a refrigerant, such as, liquid helium or liquid nitrogen or a cooling device. In order to fully use the potential of semiconductor devices, it is also required to cool them.
Furthermore, in order to operate functional devices at room temperature in a wide range of ambient temperature, thermal isolation, ventilation, and/or other mechanisms are required. In a conventional technique disclosed in Japanese Unexamined Patent Publication No. Sho. 64-17456, a semiconductor device or a transmission circuit is disposed on a cooling electrode of a Peltier device thereby controlling the temperature of the semiconductor device or the transmission circuit.
In another technique disclosed in Japanese Unexamined Patent Publication No. Hei.6-085122, a Peltier device for cooling a functional device is disposed on a radiation base plate, and the functional device is enclosed together with the cooling Peltier device in a package.
Furthermore, another Peltier device is disposed on the outer side of the radiation base plate thereby controlling the temperature of the radiation base plate. The temperature of the functional device is maintained within a desirable range by controlling the temperature of the radiation base plate at a proper value regardless of the variation in the ambient temperature.
In FIG. 1, a scanning circuit 1502 is formed on a semiconductor substrate 1501. Further, it a thermal-to-electrical transducer (or a converting device) 1506 is formed over the scanning circuit 1502 via a cavity 1504. A diaphragm 1503 is placed on the thermal-to-electrical transducer. Moreover, an infrared ray absorption layer 1505 is formed on the diaphragm 1503.
A first problem to be solved by the present invention is that when a functional device, having a peculiar physical property which appears at a particular temperature, is used, a temperature control device, such as, a cooler or a heater occupies a much greater space than the functional device itself. Furthermore, electric power consumed by the cooling device is much greater than electric power consumed by the functional device. This is a serious problem encountered when a superconducting device is used.
In high-speed signal processors, copper interconnection wires are used to reduce the resistance of interconnection wires thereby reducing the propagation delay due to parasitic CR (capacitance and resistance) associated with interconnection wires. If superconducting interconnection wires having no resistance are used, a processor capable of operating at an ultra-high speed can be realized.
In microwave devices, on the other hand, a superconducting SIS detector and/or a superconducting interconnection allow extremely low noise detection of a microwave. However, in any case, the problems are the large size and large power consumption of the temperature control device.
Similar problems also occur in devices, other the superconducting devices, which are used at a cooled temperature. More specifically, a quantum infrared sensor formed of mercury cadmium tellurium or platinum suicide is cooled using a stirring cycle cooler or a Peltier device.
However, even the most advanced small-sized low-power stirring cycle cooler has a volume as large as several hundred cm.sup.3 and a weight as heavy as 1 kg and needs electric power as high as 10 W. This example of the cooler is used when a device having a size of a few mm square is cooled at about 77K. When the device has a greater size and/or when it is required to cool the device at a lower temperature, the volume, the weight, and the power consumption of the cooler become greater.
Although the coolers using a Peltier device, disclosed in Japanese Unexamined Patent Application Publication Nos. 6-085122 and 64-17456 cited above, are smaller in volume and weight than stirring cycle coolers, there is no significant difference in power consumption.
In quantum infrared sensors and compound semiconductor devices, the electron mobility increases and noise decreases with decreasing temperature. Therefore, if such a device is operated at a low temperature, it is possible to achieve extremely low noise performance in a microwave range. However, the above-described problem also occurs in this case.
On the other hand, the dielectric constants and pyroelectric coefficients (temperature dependence of spontaneous polarization) of ferroelectric materials increase to very large levels at temperatures near their Curie temperatures.
If a ferroelectric material is used at a temperature near its Curie temperature to obtain a high dielectric constant, it is possible to realize a high-capacity memory with a small cell size.
The large pyroelectric coefficient of a ferroelectric material allows an improvement in sensitivity of a pyroelectric infrared sensor. However, also in these cases, the problems are the large size and large power consumption of devices. If a conventional temperature control device is used to cool a plurality of functional materials at different temperatures, the size of the temperature control device becomes still greater.
Even when a cooler using Peltier devices is employed, it is required to dispose as many Peltier devices each having a size of a few cm square as the number of temperature levels at which functional devices are cooled.
A second problem to be solved by the present invention is that it takes a long time for the conventional temperature control device to reach a desired temperature. Even in the case of a Peltier device which can reach a desired temperature in a rather short time, a period of time of the order of several sec to several ten sec is needed to reach a desired temperature.
The time needed before reaching a desired temperature depends on the thermal time constant of the device to be cooled and that of the Peltier device itself and also depends on the cooling ability of the Peltier device.
The thermal time constant is given by the product of the heat capacity of the device or the Peltier device and the thermal resistance between the device or the Peltier device and the outside. The thermal resistance is generally set to a rather large value so that the device can be cooled with low electric power. However, this causes an increase in the thermal time constant given by the product of the large thermal resistance and the heat capacity of the device or the Peltier device.
The large thermal time constant results in a problem that it takes a long time for a device to become operable after turning on the power to the device, and also results in a problem that a long waiting time is required to change the temperature.
A third problem to be solved by the present invention is low accuracy in controlling the temperature using the conventional temperature control device. This problem is particularly serious in temperature control devices using compression and expansion of gas, such as, a stirring cooler. Even if a conventional cooler using a Peltier device is employed, it is not easy to accurately control the temperature.
This is because the signal output from the temperature sensor responsible for monitoring the temperature is very small in magnitude and thus the signal tends to be affected by noise and also because a large current flowing through the Peltier device tends to generate noise.